Wet snow avalanche activity in the Swiss Alps - trend analysis for mid-winter season

During the winter 2011-2012 the Alps experienced repeated periods with high activity of wet snow and full-depth glide avalanches during mid-winter season (December to February). Damage to infrastructure but also fatalities were the consequence. Looking back 5 years, there was at least one intense and widespread wet snow and full-depth glide avalanche period in four out of five mid- winter seasons. To study the long-term trend, changes in wet snow and full-depth glide avalanche ac- tivity during mid-winter season were analyzed from 1952 to 2013, based on long-term observation sta- tions. Robust time series analyses showed a positive trend in number and proportion of wet snow and full-depth glide avalanche records. The trend coincides with the trend in increasing air temperature in Switzerland. A break in the data series is shown that originates probably from a major revision of the snow and avalanche recording system in 2002. For the 50 year period before the revision, the propor- tion of wet snow avalanches increased by 0.4% per year. With the anticipated increasing temperatures in the European Alps, the positive trend in wet snow avalanche activity will most likely continue, which requires adaptations in risk management.